The overall objective of this program is to achieve a better understanding of the structure and function of eukaryotic chromosomes. Our approach to this problem involves the use of polytene chromosomes as a model for interphase chromosomes in higher organisms. Three approaches will be employed in the execution of the project during the next year: 1) Studies of an alkaline active deoxyribonuclease in the salivary gland. The regulation of enzyme induction will be studied in the case of an enzyme that has been associated with a specific chromosomal puff. The effect of hormones and mutation on gene regulation will be studied. 2) The influence of BUdR on the structure and function of polytene chromosomes will be investigated. Puff induction and the stability of the polytene chromosomes will be studied in chromosomes that have been substituted with BUdR. 3) The forces that are responsible for pairing of homologous chromosomes are unknown. A search will be made for mutants which affect these forces. BIBLIOGRAPHIC REFERENCES: Elgin, S.C.R. and J.B. Boyd. 1975. The proteins of polytene chromosomes of Drosophila hydei. Chromosoma 51:135-145. Boyd, J.B. 1975. Isolation and experimental manipulation of polytene nuclei in Drosophila. In Methods in Cell Biology, Ed. D.M. Prescott, Vol. X, Academic Press, N.Y.